Carrier lifetime is one of the most important characteristics of semiconductor devices. The measurement of the carrier lifetime can be used as a monitor of the quality of epitaxial films, because it provides information about the point defect density in the semiconductor. In order to reduce the on-state resistance, the carrier lifetime must be high. A long carrier lifetime is required for effective conductivity modulation. But too long a lifetime will cause considerably long reverse recovery, leading to limited switching frequency and excessive switching loss. Therefore, carrier lifetimes need to be controlled to achieve optimum lifetime values depending upon the final application. It is therefore very important to have a controlled process where we can achieve a short or long lifetime depending upon the application. However, no method for controlling or improving the carrier lifetime using the Chemical Vapor Deposition (CVD) has been reported.
In fact, even today's high quality SiC epilayers still contain impurities and intrinsic defects acting as carrier traps or recombination centers. These defects degrade the minority carrier lifetime that is required to achieve a low on-state voltage drop for high voltage bipolar power devices. CVD growth in SiC is generally performed using silane and propane, with SiC-coated graphite parts used as the material for the susceptor and hot wall. However, SiC-coated graphite susceptors release impurities like Titanium (Ti), Boron (B), and Aluminum (Al) during CVD growth, and these impurities are electrically active, acting as traps, and thus degrading the carrier lifetime. It has also been suggested that susceptor material plays a critical role in reducing the background impurity concentration since the aforementioned impurity also follows the site-competition principle.
For high voltage bipolar devices, the minority carrier lifetime determines the level of base modulation and, consequently, the residual voltage drop across a device at high current densities. High voltage SiC power devices possess great promise in terms of lower on-resistance owing to the effect of conductivity modulation. However, the lifetime of SiC epilayers typically falls within the range of <0.05-5 μs. Thus, there is a need to increase the carrier lifetime in as-grown epilayers and to reduce the forward voltage drop. However, it is not desirable to have a lifetime that is too long because it will cause considerably longer reverse recovery, thereby limiting the switching frequency. Furthermore, long lifetimes will also cause excessive switching losses in the case of unipolar devices.
As such, a need exists for the ability to vary the carrier lifetime in as-grown epilayers in order to form an epilayer having the desired carrier lifetime.